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Distinguishing Hisingerite from Other Clays and Its Importance for Mars

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Distinguishing Hisingerite from Other Clays and Its Importance for Mars 45th Lunar and Planetary Science Conference (2014) 2251.pdf DISTINGUISHING HISINGERITE FROM OTHER CLAYS AND ITS IMPORTANCE FOR MARS. R. E. Milliken1 and D. L. Bsh2, 1Dept. Geological Sciences, Brown University, Box 1846, Providence, RI 02912 [email protected]; 2Dept. Geological Sciences, Indiana University, Bloomington, IN 47405. Background: Orbital visible-near infrared (VIS- Methods: To date we have acquired and ana- NIR) spectrometers have detected numerous occur- lyzed 10 samples purported to be hisingerite from a rences of surface deposits and exposed strata on Mars wide range of localities (North America, Europe, South exhibiting absorptions consistent with the presence of America, Australia). Five samples were provided by T. clay minerals [1-5]. In the majority of cases the ob- Eggleton from his detailed study [11] and have been served absorptions are indicative of Fe and/or Mg-rich previously measured using a variety of techniques, clays such as saponite, nontronite, chlorite, serpentine, including XRD. Two of these samples are ‘type exam- or mixed-layer smectite/chlorite [1-5]. Specifically, ples’, though it is unclear if they are the same samples these identifications rely on M-OH absorptions in the originally analyzed by Hisinger [11]. Additional sam- 2-2.5 µm wavelength region, often in conjunction with ples were acquired through mineral dealers (3), from H2O features at 1.9 µm in the case of reported smec- the Caltech mineral collection (1), and from the Indiana tites. Though significantly weaker, diagnostic absorp- University mineral collection (1). All samples were tions near ~1.4 µm due to OH and/or H2O are also ob- powdered, dry sieved to <45 µm, and measured under served in spectra for some deposits. ambient lab conditions. In addition to their indication of aqueous processes, XRD patterns were acquired at IU on a Bruker D8 the detection of clay minerals on Mars is of great sig- Advance diffractometer with Cu Kα radiation. Reflec- nificance because their fine-grained nature can promote tance spectra were acquired in RELAB at Brown Uni- trapping and binding of organic matter and the layered versity from 0.3 – 25 µm using a combination of a cus- structure and swelling capacity of smectites may have tom bi-directional spectrometer (0.3-2.6 µm) and an made them important in the evolution of complex or- FTIR spectrometer (Thermo Nicolet iS50) with a dif- ganic compounds [e.g., 6]. Indeed, the presence of clay fuse reflectance attachment. Spectra were processed by minerals was a significant factor in landing site selec- fitting a continuum slope over the wavelength region of tion for the Curiosity rover, and similar considerations interest using an upper convex hull fit and dividing will likely arise for the Mars 2020 rover. each spectrum by its corresponding continuum. The presence of clay minerals on Mars was defini- tively confirmed by in situ X-ray diffraction patterns acquired by the CheMin instrument on the Curiosity rover in Yellowknife Bay [7]. These data showed evi- dence for Fe-saponite (smectite) in ancient martian mudstones, but another significant component of the samples was XRD-amorphous. Similarly, XRD pat- terns of soil scooped from the modern Rocknest sand shadow also indicated a significant amorphous compo- nent. Though not definitive, this component may in- clude a contribution from hisingerite [8]. The potential for both smectite and hisingerite to be present on Mars in materials of vastly different age is worthy of addi- Figure 1. XRD patterns for type examples of hisignerite tional study given their different formation conditions (black: Gillinge; blue: Riddarhytten) and an additional ‘clean’ and implications for current water storage and cycling. sample (red: Mexico sample from IU collection). Sharp peaks The definition and identification of hisingerite has a are due to minor impurities. complicated history, with many studies suggesting it is noncrystalline, a precursor to nontronite, or a Results: Our XRD data of the 5 samples pre- nontronite-like clay [9-11]. However, detailed charac- viously studied and provided by T. Eggleton confirm terization by [11] showed conclusively that hisingerite those earlier results. The hisingerite samples from is not similar to nontronite (a 10Å 2:1 structure) and is Gillinge and Riddarhytten are type examples and are instead a hydrated, ferric, kaolin mineral (a 7Å 1:1 quite pure; these samples should be used as the basis structure) with a curved morphology. Thus it may be for comparision to determine if other samples contain considered the Fe equivalent of halloysite. true hisingerite. The sample from the IU collection The goal of this study was to charcterize the XRD (origin Mexico) exhibits an XRD pattern that is nearly and VIS-NIR reflectance properties of hisingerites and identical to these type examples and is also quite pure how they compare to Fe-smectites. We discuss the im- (Figure 1, red). Though not sharp, peaks in hisingerite portance and feasibility of distinguishing these phases XRD patterns are clearly distinct and can be relied up- on Mars from orbital data and address the implications on for accurate identification if several can be identi- for the presence of hisingerite on Mars. fied in a sample. 45th Lunar and Planetary Science Conference (2014) 2251.pdf Of the remaining samples, some contain hisingerite but none are as pure as the type examples. For so- called ‘hisingerites’, XRD patterns reveal that many contain smectite, chlorite, mixed-layer chlo- rite/smectite or other non-clay phases. As noted by [11], the occurrence of Fe-clays such as nontronite mixed with hisingerite is a likely reason for earlier re- ports of hisingerite having nontronite-like qualities. Though hisingerite and impurities can be readily discriminated in XRD patterns, unambiguous identifi- cation of hisingerite in such mixtures is difficult using VIS-NIR spectroscopy. As shown in Figure 2, the type examples of hisingerite have strong H2O features at 1.4 and 1.9 µm with weaker Fe-OH features at 2.28 µm and longward. The center position of the primary Fe- OH vibration at 2.28 µm is identical to that of Fe- smectites (e.g., nontronite). Therefore, analysis based solely on band position would suggest that hisingerite and nontronite cannot be distinguished at VIS-NIR wavelengths. However, the Fe-OH band at 2.28 µm is distinctly broader than observed for smectites (Figure 2), and this greater width seems to be a defining spec- tral characteristic of hisingerite. Figure 2. VIS-NIR reflectance spectra of ‘hisingerite’ sam- Reflectance spectra of other (impure) samples that ples (and notronite for comparison). Sm=smectite, mix- contain hisingerite are more ambiguous and could be layer=chlorite/smectite, his=hisingerite, serp=serpentine. confused as mixtures of other clay minerals or clay/non-clay mixtures; identification of hisingerite in ously proposed [12]. It has physical properties that are some of these samples (whose presence is verified by quite distinct from smectites, including a vitreous luster XRD) is problematic. However, the samples from Cali- and greater hardness. In addition, the hydration behav- fornia, Sweden, Saxony, and Quebec all contain smec- ior of hisingerite is distinct from smectites in that it tite or chlorite/smectite (from XRD) and exhibit reflec- does not hold H2O in interlayer regions, but instead has tance properties expected for these phases. In sum- a curved or spherical morphology and likely traps wa- mary, relatively pure hisingerite can be distinguished ter in the spheres [11]. from nontronite and other Fe-rich clays, but hisingerite Little work has been done on the dehydra- that is subordinate to other clay minerals in an assem- tion/rehydration properties of hisingerite, and it is un- blage would be difficult to identify at CRISM and clear if it would exhibit irreversible dehydration as OMEGA wavelengths. does halloysite. This is in contrast to the rehydration behavior of smectites, and limited rehydration would Conclusions: Our results show that the similarity have important implications for atmospheric exchange in reflectance properties between hisingerite and if hisingerite is present in Martian soils. Hisingerite on nontronite could easily lead to their misidentification Mars could also indicate low degrees of water-rock on Mars. Center positions of absorption bands are in- interaction, and previous work indicates that it may sufficent for discrimination and care must be used to mature or alter to Fe-saponite under elevated tempera- examine the width of the primary Fe-OH absorption at tures [10]. Ongoing work is focused on analyzing re- ~2.28 µm. In this context, the signal to noise, spectral ported CRISM detections of ‘Fe-smectite’ in detail to resolution, and spectral sampling for any given spec- determine if some may in fact be more consistent with trum must be taken into account for accurate identifica- hisingerite. tion. These issues are likely not problematic for high- resolution lab instruments but may be a concern for References: [1] Poulet et al. (2005), Science, 438, 623-627; orbital OMEGA and CRISM data. In some cases (e.g., [2] Mustard et al (2008), Science, 454, 305-309; [3] Ehlmann few pixels) data may be insufficient to differentiate et al. (2011), Nature, 479, 53-60; [4] Carter et al. (2013), between hisingerite and nontronite. Hisingerite has JGR, 118, 831-858; [5] Milliken and Bish (2011) LPSC 42, distinct peaks in XRD patterns that should be identifia- #2230; [6] Cairns-Smith & Hartman (1986), Cambridge Univ. Press, 208 pp.; [7] Vaniman et al. (2013), Science, doi: ble in CheMin data on Curiosity if it is present in suffi- 10.1126/science.1243480; [8] Bish et al. (2013), Science, cient amounts. 341, doi: 10.1126/science.1238932; [9] Whelan & Goldich Hisingerite can form by weathering, deuteric altera- (1961), Am. Min, 46, 1412-1423; [10] Shayan et al. (1988), tion, or hydrothermal alteration of mafic materials, and Clay Clay Min., 36, 327-336; [11] Eggleton & Tilley (1998), for these reasons its existence on Mars has been previ- Clay Clay Min., 46, 400-413; [12] Burns,R.
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